1. Field of the Invention
The present invention relates to a switching regulator, and more specifically to a protection circuit for a switching regulator.
2. Description of the Prior Art
Verious types of stabilized power supply circuits have been proposed and put in practical use. Of late, some types of stabilized power supply circuits employ a power transistor. If such stabilized power supply circuits undergo an overload, for example, because of short circuiting in the load, or the like, some circuit components such as a power transistor are liable to be overly driven and damaged. If such components are damaged, these must be replaced.
For the purpose of protecting the circuit from such an overload in switching regulators, various types of protection circuits have also been proposed and put in practical use in switching regulators. FIG. 1 shows an ideal characteristic curve of such a protection circuit in a stabilized power supply circuit, wherein the ordinate indicates an output voltage while the abscissa indicates an output current. Referring to FIG. 1, an ideal characteristic curve of a stabilized power supply circuit with a protection circuit comprises a substantially horizontal portion A representing a constant or stabilized output voltage irrespective of the output current and an oblique portion B extending from a given critical output current point C representing a proportional decrease both in the output voltage and the output current. The constant region A shows an ideal charactertistic curve which is required as a constant or stabilized voltage source, while the oblique portion B shows an ideal characteristic curve which is required as a protection circuit for protecting the circuit from an overload whenever the output current exceeds the output current point C.
As microelectronics was recently developed, various electronic equipment was accordingly miniaturized. On the other hand, implementation of compact electronic equipment raised a requirement of a small sized stabilized voltage supply circuit. A switching regulator has been proposed and put in practical use to provide a compact stabilized voltage supply circuit.
FIG. 2 shows a block diagram of a typical conventional switching regulator. Referring to FIG. 2, the switching regulator shown comprises a recitifier 1 coupled to receive an AC output from a commercial AC power supply. The rectifier 1 typically comprises a bridge connected four diodes, as well known to those skilled in the art. The output of the rectifier 1 is shunted by a capacitor which serves as a smoothing circuit. The output of the rectifying and smoothing circuit is connected through a primary winding of a high frequency transformer 3 to a switching transistor 2. The secondary winding of the high frequency transformer 3 is connected to another rectifying and smoothing circuit 4 for the purpose of converting the output of the high frequency transformer 3 into a direct current output. The input electrode of the switching transistor 2 is connected to receive the output from a switching control circuit 6. The switching control circuit 6 is connected to receive through a photocoupler PC the said direct current output of the rectifying and smoothing circuit 4 coupled to the secondary winding of the high frequency transformer 3. The switching control 6 is structured to be responsive to the said direct current output of the rectifying and smoothing circuit 4 to generate a train of high frequency pulses the pulse width of which is modulated as a function of the said direct current output of the rectifying and smoothing circuit 4. To that end, the switching control circuit 6 typically comprises a high frequency reference pulse generator for generating a train of reference pulses of a high frequency such as 20 to 50 KHz and a pulse width modulator for modulating the pulse width of the reference pulses as a function of the direct current output from the rectifying and smoothing circuit 4 received through the photocoupler PC. The high frequency transformer 3 typically comprises a ferrite core. The said direct current output of the rectifying and smoothing circuit 4 is withdrawn as a stabilized direct current output voltage. The switching control circuit 6 is connected to be energized by a stabilized direct current voltage. To that end, typically a voltage dividing circuit comprising a series connection of a resistor and a Zener diode is connected to the output of the rectifier 1 and the junction of the resistor and the Zener diode in the said series connection is connected to the switching control circuit 6 for the purpose of energization.
In operation, the AC output obtainable from the commercial AC power supply is rectified by the rectifier 1 and is smoothed by the capacitor. The output from the rectifier 1, as smoothed, is voltage divided and stabilized by the said series connection and is applied to the switching control circuit 6 for the purpose of energization. As a result, the switching control circuit 6 is operative to generate a train of high frequency switching control pulses to the input electrode of the switching transistor 2. As a result, the switching transistor 2 is on/off controlled in response to the switching control pulses. Accordingly, a current flows through the primary winding of the high frequency transformer 3 intermittently. The duty cycle of the current flowing through the primary winding of the high frequency transformer 3 is determined by the switching operation of the switching transistor 2. Since a current flows through the primary winding of the transformer 3 intermittently at the above described high frequency, a corresponding high frequency alternate current output is obtained across the secondary winding of the high frequency transformer 3. The high frequency alternate current output is rectified and smoothed by the rectifying and smoothing circuit 4, thereby to provide a direct current output. To direct current output is applied through the photocoupler PC to the switching control circuit 6 as a control signal. The switching control circuit 6 is responsive to the direct current output to modulate the pulse width of the reference pulses to a proper value. More specifically, if and when the direct current output voltage increases, the increase in the voltage as applied through the photocoupler PC to the switching control circuit 6 causes the width of the switching control pulses to be reduced, thereby to decrease the duty cycle of the current flowing through the primary winding of the transformer 3 and accordingly decrease the direct current output voltage from the rectifying and smoothing circuit 4, and vice versa. As a result, a stabilized or constant direct current volatage is obtained from the rectifying and smoothing circuit 4.
Now consider a case where a protection circuit is incorporated in the FIG. 2 switching regulator for the purpose of achieving an ideal protecting operation as shown in the FIG. 1 graph. To that end, simply a circuit for detecting an overload could be coupled to the rectifying and smoothing circuit 4 of the secondary side of the transformer 3 which could be coupled through another photocoupler to the control input of the pulse width modulator in the switching control circuit 6, such that the pulse width could be reduced responsive to an overload. In fact, such a circuit configuration could achieve a desired protection characteristic. Nevertheless, it has been found that such a circuit configuration requires several expensive components including the said additional photocoupler for the purpose of isolating the primary and secondary sides of the high frequency transformer 3 in conjunction with the protection circuit. Hence, there is room of improvement in such a circuit configuration.
For the purpose of avoiding the above described shortcomings, a resistor could be connected in series with the switching transistor 2 for the purpose of detecting an overload in terms of an increase in the primary current rather than in terms of an increase in the secondary output voltage. However, such an approach makes it difficult to detect a change in the secondary output voltage. Hence, according to such an approach, whenever an overload occurs, the switching control circuit 6 is controlled such that a primary current flowing through the switching transistor 2 is limited to a given constant current, or alternatively the switching transistor is controlled to be in a non-conductive state. However, in the former case where the switching control circuit 6 is controlled such that the primary current flowing through the switching transistor is limited to a given constant value, the switching transistor 2 is kept in such a constant limit current when an overload continues, resulting in an increased collector loss in the switching transistor 2 during the protecting operation and hence an overheat of the switching transistor in case of a continued overload state. On the other hand, in the latter case where the switching transistor is controlled to be in a non-conductive state in response to an overload, the switching transistor 2 is interrupted even in response to an instantaneous overload, which necessitates turning on again of a power supply switch for the purpose of resetting the circuit each time the circuit is interrupted in response to an overload. Hence, it is desired that an improved switching regulator is provided wherein the above described disadvantages have been eliminated.